Insulation for silicon steel



United States Patent 3,150,015 INSULATION FOR SILICON STEEL Charles D. Boyer, Natrona Heights, and Clarence L. Miller, Jr., Pittsburgh, Pa, assignors to Allegheny Ludlum Steel Corporation, Brackenritlge, Pa., :1 corporation of Pennsylvania No Drawing. Filed Aug. 29, 1961, Ser. No. 134,564 12 Claims. (Cl. 148-6.2)

This invention relates to improvements in coatings for steel and relates in particular to improvements in insulating coatings for electrical grades of silicon steel strip or sheet.

The use of coatings as applied to electrical grades of iron-silicon alloy strip or sheet that is to be subsequently fabricated into laminated structures for use in magnetic cores for power and distribution transformers, etc., to reduce interlaminar current loss by insulating the laminations, is a standard practice in the manufacturing of such electrical steels. Such coatings may be either organic or inorganic, but the particular coating herein presented belongs to the latter group. Inorganic coatings are generally characterized by a high degre of electrical insulation and the ability to withstand stress relief annealing temperatures and atmospheres without loss of insulation quality.

Inorganic insulating coatings may be applied to bare metal surfaces or to silicon steel strip or sheet which has been previously coated and heat treated. For example, the strip material may have been coated with MgO or CaO powders which act as a separator of adjacent coil surfaces during box annealing of the strip in coiled form or all previous heat treatments may have been conducted in a continuous manner, in which event no previous coatings would have been applied. Also, such previous coatings may have been stripped from the steel to avoid excessive Wear on punch and die sets used to fabricate laminations from such material. In any event, insulation coatings such as are presently contemplated are compatible with either the previously coated or the uncoated surfaces.

The qualities considered desirable for such coatings include the ability to insulate or retard electrical currents before and after stress relieving anneals, the effect such coatings have on the magnetic characteristics of the steel, the adhesion qualities of the coating, and the stability of the coating to environmental conditions.

Commercially available inorganic coatings include chromium trioXide-silica (or silicate) coatings such as are revealed in United States Patent No. 1,946,146 to Kiefer et al. and 2,472,596 to Kiefer and the phosphate coatings such as are revealed in United States Patents Nos. 2,492,682 to Gilford and 2,753,282 to Perry. The present invention is directed to the discovery that a greatly improved chromium trioxide-silica coating may be obtained by employing particularly finegrained silica in colloidal form in place of the conventionally available granular product.

It is, therefore, the object of the present invention to provide an inorganic coating for electrical grades of silicon steel strip or sheet that will provide improved electrical insulating properties.

It is a further object of the persent invention to provide an improved chromium trioXide-silica coating for ice electrical grades of silicon steel strip or sheet that will provide improved electrical insulating properties.

A still further object of the persent invention is to im prove the electrical insulation between stacked laminations to reduce interlaminar electrical losses.

Other objects and advantageous features of the present invention will be obvious from the following description and claims.

In general, the present invention is directed to the substitution of extremely fine-grained silica in colloidal form for the granulated material conventionally employed in the application of chromic acid-silica-coatings to the surface of electrical grades of silicon steel strip or sheet. The present invention is particularly directed to the application of a chromic acid-silica coating of from .05 to .12 mil thickness to electrical grades of silicon steel strip or sheet by passing such steel through an aqueous solution which contains from about 6 to 23%, by weight, colloidal silica, and from about 8 to 15%, by Weight, chromic acid (CrO and baking or curing the coating. For the purpose of the present invention, colloidal silica may be regarded as the condensation polymerization product of silicic acid in particulate form having a particle size no greater than about 500 millimicrons.

The conventional slurries employed for the purpose of providing CrO SiO insulating coatings are generally composed of chromic acid (CrO dissolved in Water plus milled particles of a silica or silicate, ranging in size from about .5 micron diameter to about 7 microns, or silicates may be present in solution by employing soluble alkali silicates such as water glass. The silica employed with the slurries used in conjunction with the present invention, are only a fraction of the size of the conventional milled particles. The colloidal particles may be as large as 500 millimicrons diameter, but if larger particles are employed, it may not be possible to maintain them in a colloidal state, and consequently, one does not enjoy the advantages of the present process by such a practice. Colloidal particles may be as small as l millimicron diameter. Colloidal silica is not obtained by milling conventional sources of SiO but is obtained by the controlled con densation polymerization of silicic acids. Colloidal silica therefore, as presently referred to, is not a milled mineral such as is employed in the conventional silica slurries, but is a polymer of silicic acid (presumably a polymer of monosilicic acid).

The condensation polymerization of silicic acid tends to form amorphous spherical polymers which are in a colloidal state in the liquid medium in which they are created. So long as the polymerization is discontinued before the particles grow to exceed 500 millimicrons in diameter they will continue to exist in the colloidal state.

Colloidal silica is available commercially. under the trademark Ludox (E. I. du Pont de Nemours & Co., Inc., Wilmington, Delaware), which is an aqeuous colloidal silica. The size of the particulate silica maybe as small as 7 millimicrons diameter, but may also be obtained in 17 and millimicron sizes. Colloidal silica may contain particles as small as 1 millimicron diameter. Colloidal silica and Ludox are described in United States Patent No. 2,809,137 to John Cornelius Robinson et al.

Aqueous solutions of colloidal silica and chromic acid may be made up by adding the colloidal silica in aqueous media to a prepared aqueous solution of chromic acid.

Slurries that contain less than about 3%, by weight, CrO or 6%, by weight, colloidal silica, will not effect continuous coatings with satisfactory insulating properties, particularly in the stress relieved or annealed condition.

High concentrations or thick slurries, i.e. those that contain more than about 23%, by weight, silica or 15%, by weight, C10 are difficult to apply to a sheet or strip surface and leave voids or uncoated areas which adversely affect the insulating properties of the coat.

Silicon steel (preferable in strip form) is passed through the solution or slurry preferably at room temperature, and the slurry is preferably metered onto the steel surface by rubber rolls so that a thin film of the silica-chromic acid is deposited on the strip surface, so that when the film is dried it has a film thickness of between .05 to .12

mil.

It is also preferable to conduct the coated steel when in strip form after drying continuously through a furnace to bake the coating. The temperature at which such baking takes place is preferably about 1500 F. but may be as low as 1200 F. and as high as 1700 F.

The dried and baked coating as applied will, of course, be composed of chromium trioxide and silica in approximately the same ratio as these chemicals are present in the slurry or solution. Consequently, the applied coating will be composed of from about 29 to 88%, by weight, silica and 12 to 71% by weight, chromium trioxide.

It is both desirable and preferable to dry the coating soon after its application to obtain a more uniform and evenly coated strip product. However, such drying would not be essential, since the coating would naturally dry during the baking or curing of the coated surface. Drying is conveniently accomplished by directing a stream of warm air onto the surface of the coated metal as it emerges from the coating tank.

In evaluating coatings such as are applied by the method of the present invention for their electrical insulating properties, it is conventional practice to contact both sides of the coated strip with one or more opposing electrodes between which an electric current is passed (see the instrument of U. S. Patent No. 2,982,912 to Mitchell, designed for this purpose). A measurement of the current with and without the intervening coating reflects the amount of resistivity oifered by the coated sample. The pressure applied by the electrodes is of importance, and in the test examples given below, was 900 pounds per square inch of electrode contacting area. The electric current flow between the electrodes is measured by an ammeter. At .5 volt potential, the ammeter may be adjusted so that a reading of .00 represents substantially no flow of current While a reading of 1.00 ampere represents no insulation. The readings then represent the fraction of one ampere of current permitted by the current. Such tests are called Franklin tests and are so referred to in the test results given in Tables I, II and III below. Since each test was conducted in an identical manner using the above procedure, the comparative results are significant. Such a test effectively measures the insulation quality of the coating whether it is applied to one or both sides of a flat rolled steel product so long as the testing from sample to sample is consistent. In the present testing, all samples were coated on both sides.

To conduct the tests, an aqueous slurry was prepared by mixing chromic acid, Ludox (trademark for a commercially available silica in the form of a colloidal suspension in water, property of the E. I. du Pont de Nemours Co.,.Inc.), an aqueous dispersion of 30%, by weight, colloidal SiO and water in such amounts as to effect 10%, by'weight, CrO and 13%, by weight, SiO

Samples (.014" gauge x 3 x 8") of an uncoated standard grade silicon steel strip (.02% carbon and 3.50% silicon) and of grain oriented silicon steel strip (.002 carbon and 3.50% silicon) which exhibited an extremely thin glass-like coating (less than .05 mil) which resulted Table 1 Franklin insulation tests (900 lbs./in.

As annealed As at 1,4.75 F. Type strip coated, in cracked Insolubility Adhesion amp. gas, amp.

.002 C, 3.25 Si (ori- 30 .22 Very good.... Excellent.

cnted). .02 3.25 Si (stand- .18 .31 Excellent-... Do.

Water insolubility was determined by immersing samples in clear water for 48 hour periods (room temperature) and observing any discoloration of the water. If no discoloration was observable insolubility was considered excellent; a very slight discoloration was considered good, and a slight discoloration was considered lair.

Z Adhesion was determined by bending samples over a radius of their gauge and observing any separation of the coating from the sample. If there was no observable separation at the bend, adhesion was considered to be excellent; very slight separation was considered good, and a small amount of visible separation was considered fair.

The data of Table I is significant in that under the testing procedure employed, a .4 Franklin as coated (not stress relief annealed), is commercially acceptable. In addition, as indicated in Table I, the coatings were found to have good to excellent insolubility (in water) and exhibited excellent adhesion.

Samples (3" x 8") were sheared from .018" gauge x 3" wide strip of a standard (non-oriented) (.02 C and 3.5% silicon) grade of silicon steel, part of which had been coated with an aqueous slurry made up as above, except in such proportions to effect 3.25%, by weight, CrO 13%, by weight, SiO and the balance water, and part with a slurry of 4.00%, by weight, CrO 8%, by weight, Si0 and the balance water by passing the strip continuously through the respective slurries and applying the coating by means of metering rubber rolls. The coating was dried as it left the coating tank and the strip was passed continuously through a furnace so as to bring it to a temperature of about 1200 F. for a time of about 30 seconds. Stress relieving anneals were conducted while employing various non-oxidizing atmospheres as well as still air, since the atmospheres conventionally employed vary and such atmospheres do affect the insulating qualities of inorganic coatings. Test results of the insulating properties are shown in Table II below:

Table II [Franklin insulation tests (900 lbs./in.

Coating, percent As ann., As ann., As ann., As ann., AS l,550 F. l,550 F. 1,550 F. 1,550 F. coated in cracked in forrnin nitroin still (H0, 810; gas ing gas gen air Coating, percent Insolubllity Adhesion 3. 25 13 Good Good. 4. 00 8 Excellent Fair.

As shown by Table II, the lower concentration slurries still provide good insulation properties particularly in the as coated and still air heat treated conditions, but do not exhibit the insolubility and adhesion properties of the heavier slurries.

The test results reported in Table III below were obtained from samples (about 3" x 7 sheared from .014" gauge x 12" wide coated strip (MgO separator coating applied prior to box annealing) of an electrical grade of silicon steel (about .002% carbon, 3.50% silicon and the balance essentially iron) which was coated with the aforementioned slurry (13%, by weight, colloidal silica, by weight, CrO and the balance water), by passing the strip continuously through the slurry and applying the coating by means of metering rubber rolls. The slurry contained a small amount (about 1%, by weight) of a wetting agent (Wetanol, an aqueous solution containing about 2%, by weight, modified sodium lauryl sulfate, trademark of Glyco Products Company, New York, N.Y.) The coating was dried as it left the coating tank and the strip was passed continuously through a furnace so as to bring it to a temperature of about 1500 F. for a time of about one minute. Comparative samples from similar strip (MgO coated) which was coated in an identical manner but while employing a conventional slurry of 1.8%, by weight, CrO 5.3%, by weight, Na O, 16.6%, by weight SiO (as sodium silicate), and balance water. The latter tests were conducted for the purpose of comparison. Stress relieving anneals were conducted While employing various non-oxidizing atmospheres as well as still air, since the atmospheres conventionally employed vary and such atmospheres do affect the insulating qualities of inorganic coatings.

Table III [Franklin insulation tests (900 lbs./in.

As ann.,

1,475" F. in forming amp.

As ann., ,475 F. m nitrogen, amp. a

1,475 1 in cracked gas, amp.

As coated, amp.

Coating Magneto- Electrical properties striction Coating WPP at KB 10H .ilLat fi igi rii f iiyi 1322; iiiii ii s sbdium silicate) Slog, 76.3% iiirn-A Franklin value of 0 amp. represents perfect insulation; 8. value of 1.0 amp. represents a dead short. or no insulation.

Test results shown by Table III show the coating of the present invention to provide superior insulation to the conventional coating. The improvements are particularly observable from the as coated material but also may be seen in the material stress-relief annealed in the presence of a nitrogen atmosphere and still air. Table III also shows the electrical properties of the resulting coated material. It may be observed that the watt loss (WPP) of the samples coated in accordance with the present invention is less at 15 KB and the permeability is higher at 10 H than the samples coated with the commercial coating. Also, there is shown to be less change in dimensions when magnetized in the material coated in accordance with the present invention. Hence, the present coating not only provides superior electrical insulation, but also provides excellent adhesion, is insoluble in water and does not interfere with the electrical properties of the steel to which it is applied.

We claim:

1. The method of providing an inorganic insulating coating to flat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 6% to 23%,

6 by weight, of colloidal silica and 3% to 15 by weight, CrO so as to leave a thin coating of said slurry on said surface and baking said coating.

2. The method of providing an inorganic insulating coating to fiat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 6% to 23%, by weight, of silica having a particle size not greater than about 500 millimicrons diameter and 3% to 15%, by weight, CrO so as to leave a thin coating of said slurry on said surface, and baking said coating at a temperature of from about 1200 F. to 1700 F.

3. The method of providing an inorganic insulating coating to flat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 6% to 23%, by weight, colloidal silica and 3% to 15%, by weight, CrO so as to leave a dry weight coating of said slurry on said surface of from .05 to .12 mil thick and baking said coating at a temperature of from about 1200 F. to 1700 F.

4. A composition for the coating of electrical grades of flat rolled steel comprising an aqueous slurry consisting of from 6 to 23%, by weight, of colloidal silica, 3 to 15%, by weight of CrO and the balance Water.

5. A composition for the coating of electrical grades of flat rolled steel comprising an aqueous slurry consisting of from 6 to 23%, by weight, of silica having a particle size not greater than about 500 millimicrons diameter, 8 to 15%, by weight CrO and the balance water.

6. An article of manufacture consisting of a flat rolled electrical grade of silicon steel that has been coated by contacting the surface of said steel with an aqueous slurry which contains from 6 to 23%, by weight, of colloidal silica, 3 to 15 by weight, CrO so as to leave a thin coating of said slurry on said surface, and said coating having been baked at a temperature of from about 1200 F. to 1700 F.

7. An article of manufacture consisting of a flat rolled electrical grade of silicon steel having an insulating coating on the surface thereof that is from .05 to .12 mil in thickness and which is composed of from about 29% to 88% silica and 12 to 71% CrO said coating having been applied to said steel by contacting said steel with an aqueous slurry of said components, said silica having been present in a colloidal state, so as to leave a thin coating of said slurry on the surface of said steel and said coating having been baked at a temperature of from about 1200 F. to 1700 F.

8. The method of providing an inorganic insulating coating to flat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 6 to 23%, by weight, of colloidal silica and 3 to 15%, by weight, CrO so as to leave a thin coating of said slurry on said surface, drying and baking said coating.

9. The method of providing an inorganic insulating coating to flat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 6 to 23%, by weight, of silica having a particle size not greater than about 500 millimicrons diameter and 3 to 15 by weight, CrO so as to leave a thin coating of said slurry on said surface, drying said coating and baking said coatlng at a temperature of from about 1200 F. to 1700 F.

10. The method of providing an inorganic insulating coating to flat rolled electrical grades of silicon steel which comprises contacting the surface of said steel with an aqueous slurry that contains from about 6 to 23%, by weight, colloidal silica and 3 to 15%, by weight, CrO so as to leave a dry weight coating of said slurry on said surface of from .05 to .12 mil thick, drying said coating and baking said coating at a temperature of from about 1200 F. to 1700 F.

11. An article of manufacture consisting of a fiat rolled electrical grade of silicon steel that has been coated by contacting the surface of said steel with an aqueous slurry which contains from 6 to 23%, by Weight, of colloidal silica, 3 to 15%, by Weight, CrO so as to leave a thin coating of said slurry on said surface, and said coating having been dried and baked at a temperature of from about 1200 F; to 1700 F.

12. An article of manufacture consisting of a flat rolled electrical grade of silicon steel having an insulating coating on the surface thereof that is from .05 to .12 mil in thickness and which is composed of from about 29 to 88% silica and 12 to 71% CrO said coating having been applied to said steel by contacting said steel with an aqueous slurry of said components, said silica having been present in a colloidal state, so as to leave a thin coating of said slurry on the surface of said steel and said coating having been dried and baked at a temperature of from about 1200 F. to 1700 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,030,601 McDonald Feb. 11, 1936 2,909,454 Neish Oct. 20, 1959 3,013,897 Cupery et al. Dec. 19, 1961 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No Q 3., 150 ,015 September 22, 1964 Charles D, Boyer e1; alo It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 54,, for "current", second occurrence, read coating insulation Signed and sealed this 24th day of November 1964,

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J BRENNER Attesting Officer Commissioner of Patents 

1. THE METHOD OF PROVIDING AN INORGANIC INSULATING COATING TO FLAT ROLLED ELECTRICAL GRADES OF SILICON STEEL WHICH COMPRISES CONTACTING THE SURFACE OF SAID STEEL WITH AN AQUEOUS SLURRY THAT CONTAINS FROM ABOUT 6% TO 23%, BY WEIGHT, OF COLLODIAL SILICA AND 3% TO 15%, BY WEIGHT, CRO3 SO AS TO LEAVE A THIN COATING OF SAID SLURRY ON SAID SURFACE AND BAKING SSID COATING. 